The primary problem is in yaw, not roll. The plane has no roll stability, or it is very low in frequency and is highly damped, so it will not wiggle in roll on its own. It does have yaw stability but poorly damped so it will wiggle in yaw, "fishtailing" or "snaking" on its own. Because of the dihedral effect of a swept wing, this yawing causes a secondary wiggle in roll. Stop the fishtailing and you stop the wiggle in roll, but stopping the wiggle in roll will not stop the fishtailing.

The gain between yaw and roll varies from plane to plane, some have a little yaw causing large roll and some have lot of yaw causing little roll. The former looks like it is just wiggling in roll, the latter looks like it is just fishtailing. Both will be stopped by a gyro on rudder, however with the model that has a high yaw to roll response there is more for the gyro to work with if you put it on ailerons and treat the symptom rather than the cause. That will still leave you with a wiggle in yaw which in some cases may be too small for you to notice, or a small noticeable wiggle which a yaw gyro will remove.

If you only see fishtailing, then a simple gyro on rudder is what you need, not an aileron gyro.

Do consider the quality of the gyro that you fit. Your commands from Rx to servo pass via the gyro's processor and a failure of the gyro system will usually lead to no signal getting through it, and thus a loss of control of that servo.

Harry